1. Field of the Invention
The present invention relates to orthogonal frequency division multiplication (OFDM) data transmission systems, and more particularly to methods and circuits for detecting and deciding the length of guard intervals included in received signals, and receivers including the circuits.
2. Description of the Related Art
OFDM systems are widely used and regarded as being advantageous for high-frequency data transmission through wired and wireless communication channels such digital audio broadcasts, digital television (DTV), and wireless local area networks (WLAN). OFDM is already part of WLAN, DVB (Digital Video Broadcast), and BWA (broadband wireless access) standards, and is a strong candidate for some of the fourth generation (4G) wireless telephone technologies. While a conventional (non-OFDM) communication system performs high-frequency data transmission using a single carrier (frequency), OFDM systems use multiple carriers (a multi-carrier scheme) having mutual orthogonality, so that it is possible to reduce the data rate on each carrier (e.g., by increasing the symbol period of each carrier wave) by the number of sub-carrier waves while retaining current data transmission speed, and to mitigate symbol interference due to the multi-path phenomena.
Synchronization for demodulating OFDM signals needs symbol timing sync, carrier wave sync, and sampling frequency sync. Correct synchronization is an important factor to determine the reliability of the system and the efficiency of data transmission. Since the OFDM scheme demodulates a received signal (including a succession of OFDM symbols) using a form of fast Fourier transform (FFT), it is necessary to complete a correct symbol timing sync operation to define a (symbol) period to operate the FFT.
FIG. 1 shows the structure of an OFDM symbol used in an OFDM system. The “OFDM symbol period” is composed of the useful data duration (N samples) not including the guard interval duration (e.g., G1). Useful information (e.g., WLAN transmission data) is positioned within the useful data duration (N samples), and the guard interval contains a copy of a portion of the useful data that is used as a cyclic prefix for preventing symbol interference caused in the environment of transmission.
Cyclic prefix is a crucial feature of OFDM used to combat the inter-symbol-interference (ISI) and inter-channel-interference (ICI) introduced by the multi-path channel through which the signal is propagated. The basic idea is to replicate part of the OFDM time-domain waveform from the back to the front to create a guard interval. The duration of the guard interval (the guard interval duration) Tg should be longer than the worst-case delay spread of the target multi-path environment. At the receiver side, data in the guard interval (where symbols are expected to interfere with each other by multi-path) is ignored, and the rest of the duration is demodulated by OFDM. The inserted waveform is called the “Guard Interval”, and its length is called “Guard Interval Duration”.
The data of the guard interval generally copies the (data) value of a portion of the symbol (in the useful data duration). The length of the guard interval (guard interval duration) is varied by the transmitter in accordance with the length of (expected) symbol interference (SI) generated in the environment of transmission. In order to absorb multi-path interference, the copied waveform of the OFDM symbols is inserted (in the guard interval) to effectively separate consecutive OFDM symbols by a length (guard interval) based on the expected delay of the multi-path interference. The effect of the inter-symbol interference can be minimized to assure reliable communication if the guard interval is long enough. Hereinafter, it is assumed that the useful data duration has the length of N samples and the guard interval has the length of G1, G2, G3, or G4 samples. The length of the guard interval is generally N/32(G1), N/16(G2), N/8(G3), and N/4(G4) ( i.e., a selected fraction of the length of the symbol, the useful data duration, N).
In the standard of European digital television broadcasting system, there are 2048 samples in a useful data period (one symbol), thus N=2048. Thus, it can be seen that G1=2048/32=64 samples, G2=120 samples, G3=256 samples, and G4=512 samples. The guard interval data is defined by copying a part of the useful data (symbol) and thus has a large correlation with the copied portion of the delayed useful data.
FIG. 2 is a block diagram of a conventional guard interval detection circuit. The guard interval detection circuit includes a correlator (autocorrelator) 210, a peak detector 240, a peak-to-peak interval detector 250, and a guard interval decider 260.
The correlator 210 includes a delay circuit delaying a received signal by the useful data interval size (N samples), a complex conjugating circuit 212 converting a delayed complex signal into a complex conjugate signal, a multiplier 213 performing a complex-multiplication operation on the delayed complex signals, and a sliding adder 249 adding the multiplied complex sample trains output from the multiplier within a window having the length of the minimum guard interval G1. With this construction, the conventional guard interval detection circuit obtains correlation (autocorrelation) values using the complex multiplication of received sampled data trains with the complex conjugate of N-sample delayed values of the received sampled data trains.
The delay circuit 211 is a kind of memory (e.g., shift register) delaying a sample train by a predetermined time (e.g., by N samples). The delay time is variable based on symbol length (N) for various OFDM systems.
The complex conjugator 212 converts the delayed received complex sample train into its conjugate complex value train.
The multiplier 213 performs multiplication of the non-delayed received sample train and its delayed complex conjugate train.
The sliding adder 214 adds (and accumulates) the product of the multiplication (of the received sample train and its delayed complex conjugate sample train) output from the multiplier 213, within the window having the width of the shortest guard interval length (G1), and stores the result (sums) in a memory that shifts (delays) the operation by one sample interval. If the non-delayed received sample train and the delayed complex conjugate of the received sample train are the same (high correlation), a complex sum train having a relatively large absolute value is generated from the sliding adder indicating a high correlation. Conversely, the multiplication of completely non-correlative sample trains generates a relatively small complex value.
The output of the sliding adder 214 is defined in Equation 1 as follows:
                              R          n                =                              Q                          i              =              0                                      Gl              -              1                                ⁢                      y                          n              -              i                                ⁢                      y                          n              -              N              -              i                        *                                              [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, the parameter y denotes a complex input sample, and the parameter R is a result of sliding-addition of the products (of multiplication of the non-delayed input sample by the complex conjugate of the input signal delayed by N samples) within a window having the minimum guard interval length (GI=G1). The parameter n represents an order of input samples. As the result (sum) of the sliding addition is also represented as a complex sample train, it is difficult to detect the sample size by the conventional guard interval detection circuit.
In the convention guard interval detection circuit, the length of the current guard intervals is obtained, after taking the absolute value of Rn and detecting the location of the maximum value, from operating the detected point throughout several OFDM symbols and utilizing a displacement value of the maximum value location. The guard interval's length is estimated by detecting a starting point accompanying a significant amplitude supposed to correspond to a guard interval with large correlation.
The peak detector 240 stores data, obtained from the correlator 210 in a sliding window (data frame), (in a memory) and identifies the largest stored value as a peak value.
The peak-to-peak interval detector 250 finds the maximum value position stored in the peak detector 240 and stores the detected position value in a memory.
The guard interval decider 260 detects the guard interval length by using displacement values of the maximum value position stored in the peak-to-peak interval detector 250.
Thus, the guard interval length is determined from finding a peak point having the largest correlation value by performing a self-correlation (autocorrelation) operation (correlating the input sample train with a copy of the input sample train delayed by the useful data duration of the input sample train). This utilizes the characteristic of the input signal train that the guard interval's data is set by copying a portion of the useful data and thus has large correlation with the delayed input signal train (itself, delayed).
In the conventional guard interval detector configuration of FIG. 2, detecting the guard interval length using one autocorrelation calculation and the maximum value of the result of the autocorrelation calculation, it has been difficult to assure the performance of position estimation for the maximum value, due to noises and distortions normally existing in the transmission channel. Further, as the interval including the maximum value is typically not a distinct sharp point but rather a lengthened time period (planar, platau) during the correlation calculation, it has been insufficient to estimate a guard interval length correctly. Therefore, it is necessary to increase the performance of a guard interval length estimation in an OFDM system using a guard interval affected by a channel condition, and to enhance the reliability of a timing sync in a receiver using correct detection of the guard interval.